As heating parts of planar heating elements, conventionally, parts obtained by dispersion of conductive material such as carbon black, metal powder, and graphite over resin have been known. Among those, devices using PTC (abbreviation for an English term “Positive Temperature Coefficient” that signifies positive resistance temperature characteristic) heating elements that exert a self-temperature-control function through agency of combination of conductive material and resin have been known as devices having merits such as needlessness of temperature control circuit and reduction in number of components.
In these configurations, as shown in FIG. 5, covered wire members 1 in each of which a cylindrical conductive cover 2 is applied onto an electrode wire 3 for supplying electricity to a heating resistor sheet 4 are provided, and the covered wire members 1 and the heating resistor sheet 4 are welded together by heat. The covered wire members 1 and the heating resistor sheet 4 are both formed of thermoplastic resin and conductive particles such as carbon (see Patent Document 1, for instance).
It is recommended that the covered wire members 1 should be made from the same material as that of the heating resistor sheet 4 and should each have a smooth bonding surface so that the heat welding with the heating resistor sheet 4 may be made firm.
In a planar heating element, a flat plate made of aluminum or the like is commonly applied on at least one face thereof for equalization of heat, and smoothing and thinning of the planar heating element are achieved by adoption of such a configuration as described above.
Planar heating elements of this type can be formed with small thicknesses with utilization of a characteristic thereof of needlessness of temperature control circuit and thus have been used in sites each having a comparatively thin space for installation, e.g., in floor heating systems, automobile door mirrors and mirrors of washing stands, for removal of dew and frost, and the like.    Patent Document 1: JP H03-84888 A